Independently controlled, double gate nanowire memory cell with self-aligned contacts

ABSTRACT

A doubled gate, dynamic storage device and method of fabrication are disclosed. A back (bias gate) surrounds three sides of a semiconductor body with a front gate disposed on the remaining surface. Two different gate insulators and gate materials may be used.

FIELD OF THE INVENTION

This invention relates to the field of dynamic, random-access memories(DRAMs), and devices with double gates, particularly those usingtransistors with floating bodies.

PRIOR ART AND RELATED ART

Most common DRAM cells store charge on a capacitor and use a singletransistor for accessing the capacitor. More recently, a cell has beenproposed which stores charge in a floating body of a transistor. A backgate is biased to retain charge in the floating body.

In one proposal, an oxide layer is formed on a silicon substrate and asilicon layer for the active devices is formed on the oxide layer (SOIsubstrate). The silicon substrate is used as the back gate, andconsequently, must be biased relative to the silicon layer.Unfortunately, the oxide layer is relatively thick, requiring arelatively high voltage (e.g., 100 volts) for the bias.

Several structures have been proposed to reduce this relatively highbias potential, including use of a double gate floating body and siliconpillars. These structures are difficult to fabricate. This and otherrelated technology is described at C. Kuo, IEDM, December 2002,following M. Chan Electron Device Letters, January 1994; C. Kuo, IEDM,December 2002, “A Hypothetical Construction of the Double Gate FloatingBody Cell;” T. Ohsawa, et al., IEEE Journal of Solid-State Circuits,Vol. 37, No. 11, November 2002; and David M. Fried, et al., “ImprovedIndependent Gate N-Type FinFET Fabrication and Characterization,” IEEEElectron Device Letters, Vol. 24, No. 9, September. 2003; HighlyScalable FBC with 25 nm BOX Structure for Embedded DRAM Applications ,T. Shino, IDEM 2004, pgs 265-268; T. Shino, IEDM 2004, “Fully-DepletedFBC (Floating Body Cell) with enlarged signal Window and excellent LogicProcess Compatibility;” T. Tanaka, IEDM 2004“Scalability Study on aCapacitorless 1T-DRAM: From Single-gate PD-SOI to Double-gate FinDRAM;”and U.S. patent application 2005/0224878.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a memory cell and its connection to theperipheral circuits in a memory.

FIG. 2 is a cross-sectional, elevation view of a single memory cell.

FIG. 3 is a plan view of a memory array used to illustrate metal linesand contacts to cells. Various metal layers have been striped away inFIG. 3 to assist in explaining the interconnect structure.

FIG. 4 is a perspective view of a semiconductor body formed on a buriedoxide layer (BOX), this figures includes polysilicon spacers, a siliconnitride plug, and other carbon doped silicon nitride members.

FIG. 5 illustrates the structure of FIG. 4 following the etching of thepolysilicon spacers.

FIG. 6 illustrates the structure of FIG. 5, after dry etching used toetch the BOX.

FIG. 7 is a cutaway view of the structure of FIG. 6 taken throughsection line 7-7 of FIG. 6.

FIG. 8 illustrates the structure of FIG. 7, following wet etching usedto remove the BOX under the semiconductor body.

FIG. 9 illustrates the structure of FIG. 8, after the formation of asilicon layer.

FIG. 10 illustrates the structure of FIG. 9, once spacers are etchedfrom the silicon nitride plug.

FIG. 11 illustrates the structure of FIG. 10, after a sacrificial oxidefront gate is formed.

FIG. 12 illustrates the structure of FIG. 11, following the etching backof the silicon layer.

FIG. 13 illustrates the structure of FIG. 12, once a interlayerdielectric (ILD) is deposited.

FIG. 14 illustrates the structure of FIG. 13, after the removal of thesacrificial gate and the fabrication of a front gate.

FIG. 15 illustrates the structure of FIG. 14, following the etching ofthe silicon nitride members, exposing a portion of the semiconductorbody, for source and drain region implantation.

FIG. 16 illustrates the structure of FIG. 15, following the formation ofetchant stop and dielectric layers.

FIG. 17 illustrates the structure of FIG. 16, after the etching of thedielectric layer.

FIG. 18 illustrates the structure of FIG. 17, once the etchant stoplayer is masked to reveal the front gate.

FIG. 19 illustrates the structure of FIG. 18, following the formation ofnanowire contacts to the front gate.

FIG. 20 illustrates the structure of FIG. 19, after the cell has beencompleted.

DETAILED DESCRIPTION

In the following description, a memory and method for fabricating thememory is described. Numerous specific details are set forth, such asspecific conductivity types, and metalization arrangements, to provide athorough understanding of the present invention. It will be apparent toone skilled in the art, that the present invention may be practicedwithout these specific details. In other instances, well knownprocessing steps and circuits have not been described in detail, inorder not to unnecessarily obscure the present invention.

A single memory cell is shown in schematic form in FIG. 1. A portion ofa semiconductor line or body 120, formed on an oxide layer (such as BOX250 of FIG. 2), and etched from, for example, a monocrystalline siliconlayer is illustrated. The body 120 includes a pair of spaced-apart,doped regions 110 and 130, disposed in first opposite sides of the bodythereby defining a channel region 100. In one embodiment, the channelregion is a p type region, and the source region 130 and drain region110 are more heavily doped with an n type dopant.

A pair of gates identified as a front gate 140 and back gate 150 areformed about the body 120, as will be discussed. The gates 140 and 150are insulated from the channel region 100 of the silicon body 120 by theoxide layers 160 and 170, respectively. In FIG. 1 the gates are shown onopposite sides of the body to simplify the figure. A more accuratedepiction of the cell is shown in

FIG. 2. The back gate 150 surrounds the body 120 on its opposite sidesand bottom. The front gate 140, with its nanowire contact 260, is formedabove the body. The fabrication of the cell of FIG. 2 is described inFIGS. 4-20 below. The placement of the cells in an array of cells isshown in FIG. 3, with the single cell of FIG. 2 taken through sectionline 2-2 of FIG. 3. Other cells are present in FIG. 3 at theintersections of the vertical and horizontal lines.

The memory cell of FIG. 1 is a four-terminal device, coupled to theperipheral circuits of the memory. The cell is formed in an array ofcells. For the n-channel embodiment illustrated, the source region iscoupled to ground, and the back gate 150 is coupled to a source of bias,for example, −1 volt. The drain terminal 110 is connected to a bit line230 in the memory. The front gate 140 is connected to a word line 240 inthe memory to allow selection of the cell. The cell, as will bedescribed, is a dynamic, random access memory cell, and as such, thedata stored requires periodic refreshing.

Assume first, that the cell of FIG. 1 is not storing charge, and thatthe cell is selected by the application of a positive potential to aword line which is coupled to the gate 140. (A word line 240 is shown inFIG. 3, formed from a first level of metal.) Assume further, that abinary one is to be stored in (written into) the cell as represented bythe storage of charge. (A binary 0 is represented by the absence ofcharge.) An amplifier 190 provides a positive potential to the bit line230 causing conduction in the inversion channel 210 of the channel 100of the body 120, as typically occurs in a field-effect transistor. Asthis occurs, hole pairs (resulting generally from impact ionization)drift towards the gate 150, under the influence of the bias applied tothis gate. These holes remain in the storage 200 of the body region 120after the potential is removed from the word line 240 and the potentialis removed from the bit line 230.

Assume that it is necessary to determine whether the cell is storing abinary 1 or binary 0. The cell is selected by the application of apositive potential to the word line 230. The threshold voltage of thecell shifts, depending on whether holes are stored in the region 200.The cell has a lower threshold voltage, that is, it conducts morereadily, when there is charge stored in the region 200. This shift inthreshold voltage is sensed by the sense amplifier 180 and provides areading of whether the cell is storing a binary 1 or binary 0. Thereading be provided to an I/O output line, or to refresh circuitry torefresh the state of the cell.

The threshold voltage of the cell may be determined by comparing itsthreshold voltage to that of a reference cell in a cross-coupled senseamplifier. The threshold voltage of a reference cell may be establishedby, for example, having less charge or less bias on one of the memorycells used as a reference cell.

In one embodiment, the cell is fabricated on a BOX 250 which is formedon a silicon substrate not illustrated. Active devices for the memoryare fabricated in, for instance, a monocrystalline silicon layer,disposed on the BOX 250. This SOI substrate is well-known in thesemiconductor industry. By way of example, it is fabricated by bonding asilicon layer onto a substrate, and then, planarizing the silicon layerso that it is relatively thin. This relatively thin, low body effectlayer, is used for active devices. Other techniques are known forforming the SOI substrate including, for instance, the implantation ofoxygen into a silicon substrate to form a buried oxide layer.

The described processing below focuses on the fabrication of the cellsin a memory array. While the array is fabricated on one section of theSOI substrate, the peripheral circuits for the memory may be fabricatedon other sections of the SOI substrate.

In the processing for one embodiment, first a protective oxide isdisposed on the silicon layer of the SOI substrate followed by thedeposition of a silicon nitride layer. The silicon nitride is masked todefine a plurality of spaced-apart, elongated, parallel lines and theunderlying silicon layer is etched in alignment with these lines. Theresultant structure is shown as part of FIG. 4, specifically a portionof one silicon line (body 120) disposed on the BOX 250.

A polysilicon layer is deposited over the structure and polished. Amasking and etching process is used to define the spacer 410 as well asthe plug 400. In this process, the nitride is removed from the body 120except for the plug 400 which is protected by the mask. Consequently, asshown in FIG. 4, the spacers 410 have the same width as the plug 400.Next, another layer of silicon nitride 450, doped with carbon, in oneembodiment, is deposited and polished such that it covers the exposedportions of the body 120, as well as opposite sides of the spacers 410and plug 400. These silicon nitride members 450 shown in FIG. 4, aresubsequently removed to permit ion implantation into the body 120 todefine source and drain regions.

The silicon nitride layer from which the plug 400 is formed, is atypical high temperature, dense nitride layer. On the other hand, thelayer from which the members 450 are formed is a carbon doped nitridewith, for instance, 8-12% carbon. This latter nitride etches more slowlywith a regular nitride etchant. As will be seen, this allows the removalof the plug 400 without substantially affecting the members 450.

As shown in FIG. 5, the polysilicon spacers are removed with an ordinarywet etchant, leaving substantially untouched the plug 400 and members450. This leaves an opening between the members 450 which is traversedby the plug 400.

A selective dry etching process is now used to recess the BOX 250. Asillustrated in FIG. 6, the BOX 250 is etched, resulting in the formationof a trench 480, and leaving an oxide pedestal supporting the body 250.The resultant structure is best seen in FIG. 7, a cutaway view of FIG. 6through section line 7-7 of FIG. 6. Note in FIG. 7, there remains aportion of BOX 250, a pedestal 500, under body 120.

Now, as best shown in FIG. 8, a wet oxide etchant, such as HF, is usedto etch away the pedestal 500 of FIG. 7. This etching step also resultsin the undercutting of the members 450, as shown by the undercut 490,and further thinning of the BOX 250. While in FIG. 8 it appears that thebody 120 is cantilevered from the member 450, recall that the body 120is anchored at both ends in the members 450 and rests on the BOX 250that remains under the members 450. Importantly, what has occurred asshown in FIG. 8, is the underside of the body 120 is now exposed in thechannel region of the cell, permitting the formation of the back gate onthree sides of the body 120. An oxide or other insulation is formed onthe exposed portions of the body 120 so that the subsequently depositedgate silicon is insulated from the body 120. An ordinary wet or dryoxidation process may be used to grow 5-50 Å of oxide.

Next, a low temperature, chemical vapor deposition (CVD) occurs forforming a silicon gate layer 150 both under the body 120 and along itssides where it is exposed. The silicon is, for the most part, moreamorphous than a typical polysilicon to assure deposition under the body120. A atomic layer deposition (ALD) may also be used for thisdeposition. A planarization step is used to planarized the silicon layer150 so that it is level with the upper surfaces of the members 450, asshown in FIG. 9.

A silicon nitride etchant is now used to etch the plug 400. Then, alayer of silicon nitride or carbon doped silicon nitride is deposited.This layer fills the void created by the removal of the plug 400 as wellcovering the surface of the structure. A dry (anisotropic) etchant isused to form the spacers 560, and the opening 600 between the spacers560, as seen in FIG. 10. Very little of the members 450 are etchedbecause of the relatively high amount of carbon in these members. Thisprocessing exposes the oxide layer on the upper surface of the body 120.The spacers 560 eventually provide insulation between the front gate andthe back gate.

The opening 600 is now filled with an oxide 620 (FIG. 11) using anordinary deposition and planarization step. The oxide 620, as will beseen, is a sacrificial gate, subsequently removed and replaced with aconductive gate material.

As shown in FIG. 12, the silicon gate layer 150 is etched such that theupper surface of layer 150 is below the upper surface of the oxide 620.This is done to assure that in subsequent processing a short does notoccur between the front gate and back gate once the oxide 620 isreplaced by the front gate.

A low k dielectric 650, such as a carbon-doped silicon dioxide, isformed over the structure of FIG. 12 and planarized to provide the layer650 of FIG. 13 in one embodiment.

Next, an oxide etchant is used to remove the sacrificial gate oxide 620.This exposes the upper surface of the body 120 between the spacers 560.A gate oxide is formed on the exposed body 120, for instance by growinga silicon dioxide layer 680, shown in FIG. 14. Polysilicon is thendeposited to form the front gate. CMP is used to etch back thepolysilicon layer such that the front gate does not overlay the layer650 or the spacers 450. The resultant structure is shown in FIG. 14.Rather than using a silicon dioxide insulation and polysilicon gate forthe front gate, in another embodiment, a high k dielectric is formedalong with the metal gate for the gate 140.

Now, as shown in FIG. 15, the members 450 are removed using a wetetchant such as hot phosphoric acid. Source and drain regions are thenimplanted into the exposed portions of the body 120. The formation ofthe source and drain regions may be done in two separate implantationsteps: the first to form a tip or extension source and drain region, andthe second for the main source and drain regions. This is done byimplanting the tip regions after removal of the members 450, thenforming spacers from another silicon nitride layer. These spacers areperpendicular to spacers 560; and edges of the one such a spacer isshown by the dotted line 720 of FIG. 15. The second ion implantationstep forms the main source and drain regions in alignment with thespacers 720. For the described n channel device an n type dopant such asphosphorous or arsenic is used.

Following the source and drain formation, as shown in FIG. 16, anetchant stop layer of oxide or nitride is formed. Then, an interlayerdielectric (ILD) 740 is deposited. The contacts to the underlying devicenow can be made.

To this end, first a window is opened to expose the gate 140. This maybe done with an ordinary photoresist mask which protects a portion ofthe layer 740 (see FIG. 17) allowing etching of the ILD 740 to exposethe etchant stop layer 730. This window need not be aligned as shown inFIG. 17 with the gate, but rather may be larger, as shown.

Next, as shown in FIG. 18, the etchant stop layer 730 is removed toexpose the upper surface of the gate 700. At this point in theprocessing, in an alternate embodiment, a sacrificial gate could now bereplaced with a high k dielectric and metal gate. This would be in lieuof forming the polysilicon gate 700 after the earlier removal of theoxide 620.

As shown in FIG. 19, one method of forming the front gate contact isthrough self-aligned vertical growth of metal. Selective columnar growthof carbon nanotubes (CNT) on metallic surfaces may be used.Alternatively, instead of the CNT growth, conventional contactmetallization steps could be used to complete the formation of a metalcontact without there being any penalty on the cell area. The carbonnanotube contacts 750 are shown in FIG. 19.

A second contact lithography and dry etch is used to open upsource/drain and a contact to back gate (silicon 520). This secondlithography step can be done after contact is made to the front gate700, or before contact is made to the front gate 700. The other contactssuch as the source and drain regions and back gates are not shown inFIGS. 19 and 20. They are however, shown in FIG. 3 and will be discussedin conjunction with FIG. 3. Note the contact openings are not criticallyaligned since longer openings can be made for both source/drain and backgate contacts.

The completed device is shown in FIG. 20 after a low k ILD 780 isdeposited. In an alternate embodiment, at this point in the processing,the second lithography step for contacts, mentioned earlier, could nowbe done to contact the source/drain regions and the back gate (silicon520).

One connection arrangement for connecting the cell in an array is shownin FIG. 3. In FIG. 3 some of the layers are selectively shown to allowan explanation of the connection arrangement. One such cell isrepresented by the dashed, rectangular block 800. The source of the cell800 is connected to a metal source line formed from a third layer ofmetallization (metal 3). In the adjacent line of the array, the drainsof the devices are shown connected to a metal line 230 fabricated fromthe second level of metal. Line 230 is a bit lines in the array and usedto sense the state a cell as well as write to the cell. It of courseshould be noted that there is a metal 2 bit line disposed beneath themetal 3 line formed over the cell 800 as well as over the other verticallines of FIG. 3.

The word line 240 (WL1) is fabricated from the first level ofmetallization. This line is coupled to the front gates by, for instance,the nanotubes 750. The second word line WL2 is connected to the silicon520 shown, for instance, in FIG. 20. None of the metal lines are notshown above diffusion lines 810 to reveal the source and draindiffusions. As can be seen, a single contact 830 to a source region,serves the source region of two cells and similarly, the drain contactsserve two adjacent cells. Other connection arrangements for connectingthe contacts to the lines in the array may be used.

While as described above, polysilicon (for spacers 410), silicon nitride(for plug 400) and carbon doped silicon nitride (for members 450) wereused, other combinations of material may be used for these sacrificialmaterials. Any three materials, where each of the materials may beselectively etched in the presence of the others may be used.Additionally, two of the materials should withstand the etching of theinsulative layer (BOX 250) to allow exposure of the lower surface of thebody.

Thus, a memory cell and memory have been described where a back gatesurrounds three sides of a silicon body and a top gate is disposed overthe remaining surface. By providing such complete gate coverage of thesemiconductor body, improved performance including lower leakage, andmore effective charge creation results. The low leakage provides alonger charge storage and consequently, longer times between refreshcycles are possible. The process also permits a different work functionto be used for the front gate than is used for the back gate.Self-aligned contacts are provided throughout because of the processing.Additionally, the processing permits the selection of different gatedielectrics (both composition and thickness), thereby improving the lowvoltage operation as well as the reliability of the charge storage.

1. A method for fabrication a semiconductor device comprising: formingspacers on opposite sides of a channel region of a semiconductor bodydisposed on a dielectric layer; forming a plug of a first nitridematerial above the channel region of the body; surrounding the body,spacers and plug with a second nitride material; removing the spacers todefine an opening; etching so as to remove the dielectric beneath thebody in the opening; forming a first gate beneath the body in theopening, insulated from the body; and forming a second gate insulatedfrom the body, after removal of the plug.
 2. The method defined by claim1, wherein forming the second gate comprises depositing silicon beneaththe body and on the sides of the body.
 3. The method defined by claim 1,wherein removal of the plug comprises use of an anisotropic etch to formnitride spacers from the first nitride material.
 4. The method definedby claim 1, wherein the second nitride material comprises a carbon dopedsilicon nitride.
 5. The method defined by claim 1, wherein the spacerscomprise polysilicon.
 6. The method defined by claim 1, including, afterthe formation of the first gate and before forming the second gate:etching the plug to form an opening above the channel region, theopening being defined between opposite nitride spacers; filling theopening with an oxide; removing the second nitride material; formingsource and drain regions in the body; and removing the oxide.
 7. Themethod defined by claim 2, wherein removal of the plug comprises use ofan anisotropic etch to form nitride spacers from the first nitridematerial.
 8. The method defined by claim 7, wherein the second nitridematerial comprises a carbon doped silicon nitride.
 9. The method definedby claim 8, wherein the spacers comprise polysilicon.
 10. The methoddefined by claim 1, including forming nanotubes in contact with thegate.
 11. A method for forming a semiconductor device comprising:selecting a first, second and third material, each of which may beselectively etched in the presence of the other materials; forming asemiconductor body on a dielectric; forming a plug above a channelregion of the body from the first material; forming spacers of a secondmaterial disposed on the sides of the body; surrounding the body,spacers and plug with the third material; removing the first material;etching the dielectric layer in the presence of the second and thirdmaterials without substantially etching the first and second materialsso as to expose a lower surface of the body in the channel region;forming a conductive material covering at least the lower surface of thebody, insulated from the bottom of the body; replacing the plug with agate; and removing the third material to allow the forming of source anddrain regions in the body adjacent to the channel region.
 12. The methoddefined by claim 11, wherein the first, second and third materialcomprises polysilicon, silicon nitride and carbon doped silicon nitride,respectively.
 13. The method defined by claim 11, wherein the gatecomprises metal and wherein nanotubes are grown on the gate to form agate contact.